The ability to sinter certain materials at a low temperature is extremely important to reduce the processing cost and retain desired microstructures in the materials. Certain high-strength alloys of aluminum cannot be processed using conventional powder metallurgy techniques. This is due to a high sintering temperature which results in eutectic melting and/or peritectic decomposition of the alloy, forming a non-ideal two-phase structure. Furthermore, the self-passivating nature of aluminum and other alloys leads to oxides scales on powders if exposed to air, thus inhibiting sintering. Conventional powder processing techniques rely on mechanical force, e.g. pressing or extruding, to break up the oxide scale and enable consolidation.
Reactive metal powders include those which may react violently with moisture. Examples include, but are not limited to, aluminum, magnesium, and alloys thereof. One approach to protect these metals is to deposit another material or metal on the surface. A technique for depositing a second metal is immersion deposition. Aqueous methods for immersion deposition of other metals on these materials exist, but the metals are extremely reactive and these methods are unsuitable for mass powder production due to the large release of hydrogen gas and heat produced. In addition, commercial immersion-deposition materials are generally limited to zinc and tin. These are mainly used to prepare magnesium and aluminum alloys for electrodeposition and require significant dissolution of the surface in order to obtain a thin film. This dissolution is non-ideal and would evolve a great deal of hydrogen if used with powders. Other metals such as nickel and copper can be used in immersion deposition; however, these coatings have not been commercialized due to processing difficulties and low quality of the resulting deposits on reactive metals.
These processes do not sufficiently remove the oxide barrier that inhibits sintering and they require substantial pretreatment of the material prior to deposition, often using hazardous etchants such as hydrofluoric acid. See Zelley, “Formation of Immersion Zinc Coatings on Aluminum,” J. Electrochem. Soc. 1953, volume 100, issue 7, 328-333.
There is also prior art that uses aqueous deposition to improve sintering of tungsten by creating surface eutectics. See Hayden and Brophy, “The Activated Sintering of Tungsten with Group VIII Elements,” J. Electrochem. Soc. 1963, volume 110, issue 7, 805-810, which describes that very thin layers of a carefully selected metal can drastically improve sintering. This is an aqueous process requiring thermal decomposition of the deposited salts in order to form a coating. Note, however, that tungsten is stable in water while reactive metals form more oxide when undergoing aqueous immersion deposition.
U.S. Pat. No. 6,254,757 issued Jul. 4, 2001 for “Method for electrochemical fluidized bed coating of powders,” employs a fluidized bed which maintains electrical contact between the particles and the cathode plate during cathodic electrochemical deposition of electrically activated powders. While this technique does coat the powders, it does not remove the underlying oxide barriers which will ultimately inhibit sintering of the powders.
Currently there are no known solution-based methods to deposit a variety of metals onto reactive metal powders. What is desired is a process capable of depositing thin layers of a wide variety of metals onto powders of magnesium, aluminum, and their alloys, among other metals. Safe, controllable, and convenient processes are highly desirable, such as anhydrous processes that do not require an electrical current for deposition and do not evolve hydrogen and heat. These processes should avoid intense thermal treatment for generating a final coating or sintered part.